In recent years, with advancement of miniaturization, a wafer shape that even an outer peripheral portion of a semiconductor wafer is flat has been demanded, and new indexes such as ROA (Roll Off Amount, which is also referred to as an edge roll off amount) that evaluates flatness of a semiconductor wafer outer peripheral portion, ESFQR (Edge Site Frontsurface-referenced least sQuares/Range), ZDD (Z-Height Double Differentiation) that evaluates a change in curvature, and others have been used in addition to GBIR (Global Backsurface-referenced Ideal plane/Range), SFQR (Site Frontsurface-referenced least sQuares/Range), SBIR (Site Backsurface-reference Ideal plane/Range), and others that are conventional flatness evaluation indexes.
A removal stock of the polished semiconductor wafer outer peripheral portion increases due to contact with a polishing pad, and outer periphery sag occurs as a shape after polishing. Usually, ROA or ESFQR is calculated with use of data from a semiconductor wafer outer peripheral end to a point that is 1 mm apart therefrom, and SFQR or SBIR is calculated with use of data from the semiconductor wafer outer peripheral end to a point that is 2 mm apart therefrom. Therefore, ROA or ESFQR is highly affected by the outer peripheral sag as compared with SFQR or SBIR. Further, a thickness of the semiconductor wafer greatly varies in a region from the wafer outer peripheral end to a point that is 0.5 to 1 mm from this end and, if an outer peripheral excluded region of ROA or ESFQR is further reduced from 1 mm in the future, ROA or SFQR will be more highly affected by the outer peripheral sag. A flatness index defined in the SEMI standard will now be individually described hereinafter.
GBIR is a backsurface-referenced global flatness index, and it is used for evaluation of flatness concerning an entire wafer surface defined except a peripheral edge portion. GBIR is defined as a width of a deviation between maximum and minimum thicknesses of a front side of a semiconductor wafer relative to a reference surface when a back side of the semiconductor surface is determined as a reference surface.
SFQR is a frontsurface-referenced site flatness index, and it is evaluated in accordance with each site. SFQR is defined as a range of positive and negative deviations from a reference surface when a cell having an arbitrary dimension (e.g., 26 mm×8 mm) is determined on a front side of a semiconductor wafer and a surface obtained by the method of least square in regard to this cell surface is determined as the reference surface. Further, a value of SFQRmax represents a maximum value of SFQR in each site on a predetermined wafer.
SBIR is a backsurface-referenced site flatness index. SBIR is a thickness deviation in a cell having an arbitrary dimension (e.g., 26 mm×8 mm) on a semiconductor wafer relative to a reference surface when a back side of the semiconductor wafer is determined as a reference surface, and SBIRmax represents a maximum value of SBIR in each site. SFQR and SBIR relate to evaluation of flatness of a specific cell on a wafer surface, and this evaluation is performed with respect to a cell having a dimension that approximately corresponds to a region of a semiconductor component to be fabricated.
Definition of ROA will now be described with reference to FIG. 13. An abscissa in FIG. 13 represents a length from an outer peripheral end of a semiconductor wafer, and an ordinate in the same represents a displacement of a shape of a wafer surface. In general, ROA represents, as a sag amount, a change “d” in shape displacement from a reference surface at a position that is 0.5 mm or 1 mm from an outer peripheral end (a length from the outer peripheral end of the semiconductor wafer is represented as r0 in FIG. 13) when a back side of a semiconductor wafer is corrected to a flat surface, an inclination of a front side of the semiconductor wafer is corrected, and a flat region that is 3 to 6 mm from the outer peripheral end of the semiconductor wafer (between r1 and r2 in FIG. 13) is determined as the reference surface. An outer peripheral end side of r0 is also referred to as an outer periphery excluded region (which is also referred to as a peripheral portion excluded region and corresponds to a length of a portion, which is outside the application range of the flatness standard, from the outer peripheral end).
Definition of ZDD will now be described with reference to FIG. 14. An abscissa in FIG. 14 represents a length from an outer peripheral end of a semiconductor wafer, and an ordinate in the same represents a displacement of a shape of a wafer surface. In general, ZDD means a second order differential of a displacement of a surface of the semiconductor wafer relative to a radius of the semiconductor wafer. A positive value of ZDD represents that a surface is displaced in a rising direction, whereas a negative value of the same represents that the surface is displaced in a sagging direction.
ESFQR is the SFQR at an edge (an outer peripheral portion), and it is a flatness index that is indicative of flatness of the outer peripheral portion. How to take a cell in ESFQR will now be described with reference to FIG. 15. FIG. 15(a) is a top view of a semiconductor wafer and shows that an outer peripheral portion thereof is divided into 72 rectangular regions (cells). FIG. 15(b) is an enlarged view of one of the rectangular regions, and the rectangular region is surrounded by a straight line L2 of 35 mm extending from an outer peripheral end in a diameter direction and an arc L1 corresponding to 5° in the circumferential direction of an outer peripheral portion of the semiconductor wafer, and a region of L3 that is 1 mm from the outer peripheral end in the diameter direction is not included. Here, ESFQR is an SFQR value (a range of positive and negative deviations from a least-square plane in the region) of this rectangular region (the cell). In case of ESFQR, an outer peripheral end side indicative of L3 is determined as an outer periphery excluded region.